Advanced digital cellular systems typically offer a circuit-switched data channel for data applications. As is well-known, circuit-switched connections are very heavy to establish. In addition, at least one traffic channel of the system capacity has to be allocated to the circuit-switched connection until the connection is released. For these reasons, circuit-switched data connections are suitable mainly for applications which continuously require a transfer capacity of one transfer channel of the system for relatively long periods of time. Allocation of a circuit-switched channel for the transfer is a typical example of applications suitable for the system.
Many data applications involve a low, randomly occurring transfer demand and a need to keep the (virtual) transfer connection open continuously. A circuit-switched connection is not suitable for such purposes as the resources of the cellular system are reserved unreasonably, and the implementation of a service would therefore become too expensive.
Advanced digital cellular systems also comprise a short message service. A short message may be used in data services for the transfer of small amounts of data. In the GSM system, for instance, sending a short message is similar to a conventional call set-up procedure, i.e., it is a very heavy signalling event. The applicability of short messages in the establishment of economical data connections is therefore limited.
There are prior art packet-switched radio systems implemented for data applications. Such systems require separate network investments for data connections. As the modern cellular systems as such comprise functions useful for data applications, it is advantageous to aim at utilizing these functions in providing economical data connections.